Question 1

This question is about weak acids.

Benzoic acid, $C6H5COOH\text{C}_6\text{H}_5\text{COOH}$ , is a weak monobasic acid.

Write the expression for the acid dissociation constant, $K_a$ , of $C6H5COOH\text{C}_6\text{H}_5\text{COOH}$ .

The expression for the acid dissociation constant, $K_a$ , of $C6H5COOH\text{C}_6\text{H}_5\text{COOH}$ can be simplified to:

K_a = \frac{[\text{H}^+]^2}{[\text{C}_6\text{H}_5\text{COOH}]}

State one approximation that allows the original expression to be simplified to this form.

A student carries out an experiment to determine the $pKa\text{p}K_a$ value of a solution of $C6H5COOH\text{C}_6\text{H}_5\text{COOH}$ .

The concentration of $C6H5COOH\text{C}_6\text{H}_5\text{COOH}$ is $dm−30.0800\text{ mol dm}^{-3}$ .
The pH of the $C6H5COOH\text{C}_6\text{H}_5\text{COOH}$ solution is $2.65$ .

Use the simplified expression to calculate the $pKa\text{p}K_a$ value of $C6H5COOH\text{C}_6\text{H}_5\text{COOH}$ . Give your answer to 2 decimal places.